The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. UMTS Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. As illustrated in FIG. 8, an E-UTRAN typically comprises user equipments (UE) 850 wirelessly connected to a radio base station 800.
Orthogonal Frequency Division Multiplexing (OFDM) has been adopted as the transmission scheme for the radio interface in 3GPP LTE and is also used for several other radio access technologies and standards such as Digital Video Broadcasting (DVB), Digital Audio Broadcasting (DAB), IEEE 802.11a/g (WLAN/WiFi), IEEE 802.16 (WiMAX), Hiperlan 2, and various Digital Subscriber Line (xDSL). OFDM is a frequency-division multiplexing scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. To be a bit more specific, a variant of OFDM, namely Orthogonal Frequency Division Multiple Access (OFDMA), is used for 3GPP LTE and allows different users to be multiplexed on different sets of sub-carriers. The uplink in 3GPP LTE is based on Singe Carrier Frequency Division Multiplexing (SC-FDMA), which also can be regarded as DFT pre-spread OFDM with a cyclic prefix (the use of cyclic prefix is described below). An LTE uplink sub frame is schematically illustrated in FIG. 7b. 
In traditional Frequency Division Multiplexing (FDM), different users are allocated different frequencies, or channels, for their transmission. To avoid interference between these channels the FDM frequencies must be spaced apart, which leads to a waste of frequency spectrum. In OFDM, the frequencies of the sub-carriers are chosen in such a way that they do not interfere with each other—they are orthogonal. This allows for a tighter “packing” of the sub-carriers and increased spectrum efficiency in comparison to FDM. To ensure orthogonality the sub-carriers must have a common, precisely chosen frequency spacing or sub-carrier spacing and this frequency spacing is exactly the inverse of the OFDM symbol duration. Due to its specific structure, OFDM allows for low-complexity implementation for the modulation and demodulation, by means of Discrete Fourier Transform (DFT) operations for which computationally efficient Fast Fourier Transform (FFT) algorithms exist.
As the data is divided into several parallel data streams or channels, one for each sub-carrier, the symbol rate of each sub-carrier is much lower than the total symbol rate and the sub-carrier symbol length is thus extended. This reduces the systems sensitivity to inter symbol interference (ISI) due to multipath effects (i.e. different versions of the same signal travelling different paths over the radio interface and thus arriving at the receiver at different points in time, resulting in a signal delay spread). The explanation is that ISI due to multipath depends on the relation between the signal delay spread and the symbol length, so if the symbol length is extended the system will be more robust to multipath effects.
However, although the system is more robust to multipath effects, there will still be some ISI left. This is why a guard interval between the symbols is introduced, allowing multipath to settle before the main data arrives at the receiver. Thanks to such a guard interval, there will be no ISI as long as the delay spread does not exceed the guard interval duration. A commonly used mechanism in different radio access systems, illustrated in FIG. 1, is to insert a cyclic-prefix (CP) 102 in this guard interval in front of the symbol 101. The CP is a copy of the last part 103 of the symbol 101. To avoid confusion, the symbol comprising the CP will be called the “symbol with CP” 100, to be able to distinguish it from a symbol 101 comprising only the useful part. Note that CP insertion can be used in single carrier systems as well as in multi carrier systems.
CP insertion in OFDM implies that the linear convolution inherent in the radio channel can be translated into a cyclic convolution. This cyclic convolution has the benefit to translate into an element wise multiplication when DFT or FFT transforms are considered. Moreover, this mitigates inter-channel interference among the sub-carriers.
In different systems and standards, two or more different CP duration alternatives have been incorporated to cater for the different propagation conditions. In LTE for example, a short CP to use when the delay spread is small has been specified (thus permitting low overhead), as well as a long CP when the delay spread is large (thereby sacrificing throughput somewhat).
In some scenarios, the signal delay spread might however still be larger than the defined CP duration, which would then result in ISI problems. This may for instance arise in the situations schematically illustrated in FIG. 2a-d: 
FIG. 2a: Very large cells 200a with severe multipath between the base station 201a and the UE 202a due to the long distances.
FIG. 2b: Single Frequency Networks (SFN) 200b with multiple transmitters sending the same signal from widely separated base stations 201b and 203b to one UE 202b, as for example in DVB and DAB systems.
FIG. 2c: On-frequency repeater stations (RS) 203c inducing a significant delay of the signal forwarded from the base station 201c to the UE 202c. 
FIG. 2d: Distributed antenna systems (DAS) 205d where, from a signal processing point of view, it is preferable that signals from different widely separated radio heads, 201d and 203d, are overlapping in time when received by the UE 204d. 
In systems with very large distances between transmitters and receivers, such as in cellular systems with large cell sizes (FIG. 2a), another problem is that the received power is quite significantly reduced as well.